The subject matter disclosed herein relates to a system for use in treating biogas and, more specifically, for a system located below grade to perform a separation process on biogas by which carbon dioxide is separated from methane in the biogas.
As is known in the art, biogas is produced from anaerobic digestion and contains primarily methane and carbon dioxide with lesser quantities of other constituents. Methane may be present in an amount ranging from fifty to sixty-five percent (50-65%) by volume, carbon dioxide may be present in an amount ranging from thirty-five to fifty percent (35-50%) by volume, and the other constituents may include small percentages of nitrogen, oxygen, hydrogen sulfide, and other trace constituents.
As is also known in the art, it is desirable to separate the methane from the carbon dioxide along with other constituents to obtain a purified gas that may be used as a natural gas substitute. Several processes exist by which the methane can be separated from the biogas including, for example, a water wash process, chemical absorption, pressure swing absorption, and membrane separation.
During the water wash process, biogas is injected into water relying on the fact that carbon dioxide and hydrogen sulfide are many times more soluble in water than methane. The process typically occurs at an elevated pressure and reduced temperature to enhance the solubility of carbon dioxide and hydrogen sulfide in water. Historically, tall vessels have been constructed in which water is pumped into the top of the vessel and the biogas is pumped into the bottom of the vessel. As the biogas rises through the water, the carbon dioxide, hydrogen sulfide, and other water soluble trace constituents are absorbed into the water. As previously indicated, the process is more efficient at an elevated pressure and at a lower temperature. Thus, the water is often chilled prior to entry into the vessel and the biogas is compressed prior to entry into the vessel.
However, such systems have several drawbacks. The height of the vessels is substantial in order to provide sufficient time for the biogas to be in contact with the water and for the carbon dioxide to be absorbed by the water. Further, the material from which the vessel is made must be corrosion resistant due to the presence of hydrogen sulfide in the biogas and due to carbonic acid formation from the carbon dioxide released during the process. The vessels are, therefore, typically constructed of stainless steel. The size and materials of the vessel as well as the volume of water within the vessels result in a substantial amount of weight for each vessel. Thus, the water wash treatment vessel requires a substantial physical foundation as well. The physical construction of the system as well as the materials from which the system are constructed are significant capital expenditures for a water wash treatment facility. Thus, it would be desirable to provide an improved system and method for performing the water wash process.